高真空下における高強度鋼の微小き裂から長いき裂に進展するまでの疲労き裂進展特性と破面様相

抄録

In general, the fatigue strength of high-strength steels is not high relative to their static strength, and there is no clear fatigue limit. It is also well known that in the low-stress, ultra-long-life region, internal fractures called fish-eye fractures occur. In fish-eye fractures, ODA (Optically Dark Areas), which are highly uneven fracture surfaces, are observed, and it has already been reported that the number of cycles required for their formation accounts for almost the entire life. It is essential to clarify the mechanism by which ODAs form to evaluate the life of fish-eye fractures, but this has not yet been elucidated. In this study, we therefore performed fatigue tests in a vacuum environment to simulate the internal environment on the specimen surface. We considered the relationship between the stress intensity factor and the crack growth rate until it transitioned to a long crack from the extremely slow crack under conditions where the stress intensity factor was ΔK ≤ 4 MPa√m or less. As a result, we found that small cracks grow at an extremely slow rate of 10^-15 m/cycle, and that the crack growth rate increases as ΔK increases when the stress intensity factor is ΔK ≤ 4 MPa√m. Furthermore, a clear ΔK dependency was observed when the same crack was observed continuously. In addition, after decelerating from da/dN ≈ 10^-13 at ΔK = 2.3 MPa√m, it accelerated again as ΔK increased. After reacceleration, the crack showed rapid growth behavior at ΔK≥4 MPa√m. This is thought to be due to a transition to the growth behavior of a long crack in a vacuum. Moreover, after the test piece broke, the fracture surface was observed, but ODA was not confirmed.